Although tantalum powder has hitherto been used for anodes of electrolytic capacitors, there has been a problem that tantalum is small in production and the supply price of tantalum is unstable. In these years, a trend of using niobium, abundant in amount of deposit and low in supply price, for anodes of electrolytic capacitors has been accelerated.
If the particle size of a niobium powder to be used as a material for an anode is made small for the purpose of manufacturing an electrolytic capacitor large in electrostatic capacitance, the sintering behavior in manufacturing a niobium sintered body is varied. More specifically, the temperature dependence of the shrinkage in sintering becomes large, and simultaneously the pores in the sintered body become small. Consequently, when capacitors are manufactured, deviations in electrostatic capacitance of the products are caused by the temperature distribution in a heating furnace, and filling of a conductive resin and the like to be used for cathodes become difficult. Accordingly, there are needed niobium powders small in particle size and small in the temperature dependence of the sintering behavior. Moreover, there is a problem that capacitors using niobium powders are generally larger in leakage current and larger in degradation of properties at high temperatures than capacitors using tantalum powders.
There are solid electrolytic capacitors made of valve metal powders composed of tantalum added with nitrogen, silicon, phosphorus, boron and the like (see for example, Patent Document 1). Additionally, there is a technique in which the sintering speed is suppressed by doping tantalum with phosphorus, sulfur, silicon, boron, nitrogen and the like (see for example, Patent Document 2).
In these techniques, the particles of the objective powders are relatively large, and hence these techniques cannot be applied as they are to niobium powders small in particle size to attain high electrostatic capacitance.                [Patent Document 1] U.S. Pat. No. 3,825,802 (column 2, lines 13 to 19)        [Patent Document 2] U.S. Pat. No. 4,544,403 (column 2, lines 51 to 61)        